Replace Maybe with std::optional

With c++17, std::optional provides the functionality that Maybe
provided.

Bug: 183215655
Test: aapt2_tests
Change-Id: I62bb9c2fa4985dc5217a6ed153df92b85ad9a034
diff --git a/tools/aapt2/Source.h b/tools/aapt2/Source.h
index 92934c3..4f9369a 100644
--- a/tools/aapt2/Source.h
+++ b/tools/aapt2/Source.h
@@ -17,21 +17,20 @@
 #ifndef AAPT_SOURCE_H
 #define AAPT_SOURCE_H
 
+#include <optional>
 #include <ostream>
 #include <string>
 
 #include "android-base/stringprintf.h"
 #include "androidfw/StringPiece.h"
 
-#include "util/Maybe.h"
-
 namespace aapt {
 
 // Represents a file on disk. Used for logging and showing errors.
 struct Source {
   std::string path;
-  Maybe<size_t> line;
-  Maybe<std::string> archive;
+  std::optional<size_t> line;
+  std::optional<std::string> archive;
 
   Source() = default;